1. Field of the Invention
The present invention relates to a method of measuring underground-laying depth of an electric power cable.
2. Description of the Related Art
Most of accidents of electric power cables, particularly of submarine cables, are caused by external injuries by running anchors of ships, fishing instruments such as trawlnets, dredging, or the like. It is therefore essential to lay cables underground at a predetermined depth in order to prevent those injuries. It has been found that the reliability of a cable is improved greatly if the cable is laid underground at a predetermined depth in soil.
Therefore, the following two points are important.
(1) A cable is laid underground at a predetermined depth surely when the cable is laid.
(2) The predetermined underground-laying depth is kept over the whole length of the cable during the service period of the cable.
Of these points, it is relatively easy to lay a cable underground at a predetermined depth surely when the cable is laid, but it is difficult to keep the predetermined laying depth, because the underground-laying depth cannot be kept because of sand movement (sand wave) caused by tides thereafter, changes of submarine topography caused by dredging, and the like. Accordingly, the following problems exist: (A) that the underground-laying depth becomes abnormally shallow so that injury cannot be prevented, and (B) that the underground-laying depth becomes abnormally deep so that the cable temperature nearby increases abnormally (hot spot) to deteriorate the cable.
It is therefore necessary to monitor the underground-laying depth in the longitudinal direction of the cable to make it possible to perform maintenance immediately when abnormality is found.
The conventional technique to measure the underground-laying depth of a submarine cable have been known as follows.
(1) The underground-laying depth is directly measured by a diver.
(2) A magnetic field generated by a cable in a spot is measured while a search coil is moved perpendicularly to the cable, this operation being repeated.
(3) An optical fiber is combined with an electric power cable, and an optical fiber distributed temperature sensor is connected to this optical fiber to measure the temperature distribution along the electric power cable to thereby obtain the underground-laying depth on the basis of the measurement (Unexamined Japanese Patent Publication (kokai) No. Hei-6-70435). That is, the underground-laying depth is measured by a diver or the like in advance after the cable was laid, and the change of the underground-laying depth is detected by comparison with the temperature distribution in the longitudinal direction of the cable when the cable was paid and the temperature distribution when the underground-laying depth is measured.
However, the above-mentioned methods (1) and (2) are difficult to be operated, and cannot always monitor the underground laying depth.
In addition, in the technique (3), it is difficult to detect the underground-laying depth accurately due to the influence of the temperature change of the ocean floor, thermal resistivity or the like of the soil where the cable is laid, and the like.